A determination of soil shear strength is important in a number of scientific and engineering applications. It is often sought of necessity in the areas of structural and geological engineering. Soil shear strength data, for example, may dictate whether a particular structure can be securely built at a chosen location. Or, it may influence the design parameters of a building foundation or a toxic substance landfill. It is also rather commonly used in dam construction and dredging operations. Finally, soil shear strength data is a significant consideration in earthquake prediction and analysis.
Early devices for determining soil shear strength were suitable for laboratory, rather than insitu, testing of a soil sample. It was recognized, however, that significant margins of error were sometimes introduced by the removal of the soil sample from its environment. This was particularly so in the case of dredging operations and dam construction. Consequently, it was highly desirable to have a device capable of measuring insitu the shear strength of soil.
In this context, a number of techniques have been developed. One such technique, described in U.S. Pat. No. 4,411,160 involves an apparatus having measurement devices located on the blades of a vane inserted in the soil. Here, the apparatus includes an above-surface torque motor which is connected to the vane by one or more sub-surface shafts. The vane blades have cavities in which strain gages are disposed. When the torque motor applies torque to the shafts and the vane, each gage measures the stresses acting on a portion of the corresponding blade. Afterwards, a mathematical calculation based on the respective stress measurements for each gage results in an approximation of soil shear strength.
This technique, however, has significant drawbacks. In particular, the location of the strain gages on the blades of the vane makes it relatively complex and difficult to change or replace the vane. Moreover, each strain gage simply measures the stresses acting on a portion of the corresponding blade, rather than on the vane as a whole.
Another technique, described in U.S. Pat. No. 2,907,204, involves the mounting of a strain gage to a component of the torque motor at the top of the shaft. The soil shear strength measurement is thus made at the surface. While this procedure has certain advantages compared to placing strain gages on the blades of the vane, it too has drawbacks.
In particular, it is necessary to compensate for frictional forces stemming from, among other things, the shaft support bearings and the rubbing of the soil against the rotating shaft. These forces increase as the length of the shaft increases. For best results, it is thus necessary to take frictional measurements at varying depths along the shaft to compute frictional effects.
Accordingly, there is a need for a device capable of more accurately measuring insitu the shear strength of soil, and doing so simply, economically and accurately.